It has been known for some years that a part of the starch contained in the human diet can pass the small intestine without being digested. This fraction of the food starch is called resistant starch. Different forms of starch have been found to be resistant to digestion. A classification of resistant starches has been given by Englyst and Cummings (Am. J. Clin. Nutr. (1987) 45 423-431). These authors distinguish between three types of resistant starches:
Type 1. Physically indigestible starch e.g. partially milled grains and seeds, PA1 Type 2. Resistant starch granules e.g. raw potato, green banana, PA1 Type 3. Retrograded starch e.g. cooled-cooked potato, bread, and cornflakes. PA1 a) thinning of the starch, PA1 b) enzymatic debranching of the thinned starch, PA1 c) inactivation of the enzyme, PA1 d) drying of the composition.
Effective enrichment of food with RS is possible by addition of processed starch containing a large percentage of retrograded structures. Starch is composed of amylose and amylopectin. The extent of retrogradation is known to be a function of the amylose content. Heating and cooling of amylose gives rise to resistant starch. Due to the branched structure of amylopectin the amount of resistant starch which is formed is decreasing with an increase in the amount of amylopectin in starch. The amount of RS can however be increased by debranching the amylopectin prior to heating. In view of the above high amylose (maize) starches have been chosen as the primary source of resistant starch for the first commercial high RS-products.
Carbohydrates which are not enzymatically digested in the small intestine reach the colon where they are fermented by the anaerobic microflora. Such carbohydrates include non-starch polysaccharides, resistant starch (RS), indigestible oligosaccharides and endogenous polysaccharides from mucus. The undigested starch fraction reaches the colon where it becomes a substrate for microbial fermentation. Besides gas production (H.sub.2, CH.sub.4, CO) different short chain fatty acids (SCFA) are formed depending on the type of carbohydrate.
The major end products of bacterial carbohydrate breakdown are short-chain fatty acids (SCFA: acetate, propionate and n-butyrate). SCFA are rapidly taken up by the colonic epithelial cells. Propionate and acetate are released by the basolateral membrane to the portal circulation and may have an effect far from their production site. n-Butyrate serves as energy yielding substrate in the colonocytes and additionally affects several cellular functions e.g. proliferation, membrane synthesis and sodium absorption.
Acetate, propionate and n-butyrate are the main SCFA produced from indigestible oligo- and polysaccharides the relative amounts of these fatty acids depend on the type of carbohydrate. SCFA are produced in the proximal colon in an average ratio of acetate:propionate: n-butyrate equivalent to 60:25:10 and in amounts of mmol/L. This ratio however is riot constant but is determined by the kind of substrate fermented. It has been shown both in vitro and in vivo that the fermentation of starch yields high levels of n-butyrate. The observations that ceacal SCFA levels are decreased by raw potato starch (Mallett et al. (1988) Brit. J. Nutr. 60, 597-604; Levrat et al.(1991), J. Nutr. Biochem.2, 31-36; Mathers et al., (1991) Brit. J. Nutr.66, 313-329) but increased by high amylose corn starch underline that different forms of RS have different effects in terms of n-butyrate production in the colon.
According to Wyatt and Horn (1988) J. Sci. Food Agric. 44, 281-288, RS-fractions of retrograded pea and corn starch respectively show quantitative differences in in vitro fermentation but without qualitative changes in SCFA composition. Six different raw starches also showed different in vitro fermentation kinetics. At the same time the molar n-butyrate proportion was not altered. Several independent in vivo animal studies confirm this. Thus the source of RS is important for the fermentability and hence for the amount of n-butyrate obtained but apparently not for the relative amount.
Compared with indigestible polysaccharides such as arabinogalactan, xylan and pectin, RS produces a significantly larger molar amount of n-butyrate (Englyst et. al. (1987) in I. D. Morton: "Cereals in a European Context", Chichester, UK, Ellis Horwood Ltd., pp. 221-223). This is considered important because of the general acceptance that n-butyrate plays a major role in the prevention of intestinal cancers (e.g. colorectal cancer) as recently summarised by Smith and German (Food Technology, (1995 November) 87-90). n-Butyrate appears to be a preferred substrate for normal colonocytes and assists in the maintenance of colonic integrity.
n-Butyrate inhibits growth of colon cancer cell lines. At the molecular level, n-butyrate causes histone acetylation, favours differentiation, induces apoptosis and regulates the expression of various oncogenes. In vivo n-butyrate increases immunogenicity of colon cancer cells.
Only indigestible polysaccharides which are associated with production of high n-butyrate concentrations in the distal large bowel (wheat bran, retrograded high amylose starch (type 3 RS)) were found to be protective against colorectal cancer in a rat model system wherein rats were treated with 1,2-dimethylhydrazine (McIntyre et al. (1993), Gut 34, 386-391; Young et al. (1996), Gastroenterology 110(2): 508-514). Oat bran, guar gum, raw potato starch (type 2 RS), cellulose and starch-free wheat bran have no protective effect in this model of colorectal cancer (McIntyre et al. (1993), Gut 34, 386-391, Young et al. (1996), Gastroenterology 110(2): 508-514).
From the above studies it appears that the amount of n-butyrate produced in the colon is important. What is needed for a maximal physiological benefit is not only a starch product with a high amount of RS, but a well fermentable RS-fraction producing high amounts of SCFA with an elevated n-butyrate level. Methods for the preparation of resistant starch have for example been disclosed in the following publications.
European patent application EP 688,872 discloses a method for obtaining increased levels of resistant starch. It is demonstrated that the highest amounts of RS are obtained when after enzymatic digestion, retrogradation is performed for a prolonged period of time and at a relatively low temperature. The maximum amount of RS which could be obtained was 51.8% (example 3 therein).
International patent application WO 91/07106 discloses a method for obtaining resistant starch wherein a retrogradation step is followed by enzymatic hydrolysis. The retrogradation step is performed at low temperature for amylose at 4.degree. C. and for starch at 8.degree. C. as mentioned on page 13. Moreover the process starts with undegraded starch which may be prior treated by a debranching enzyme.
European patent application EP 564893 discloses a method for obtaining resistant starch starting from a non-degraded high amylose starch. The DSC melting peak temperature of this product is mentioned to be in the range of 115-135.degree. C. and the amount of resistant starch is below 51% and is correlated with the percentage of amylose used in the starting product.
There exists a need for a starch-based product which is highly fermentable and which gives rise to an increased amount of n-butyrate in the colon. The present invention provides such a starch-based product.